Companion crops in winter oilseed rape : effects on nitrogen concentration, biomass, and prevalence of weeds at the end of autumn growth

Abstract: Intercropping can be used as a management system to promote higher yield as well as less weeds,diseases, and insect damage. Legumes are a commonly used companion crop (CC) in intercropping due to the benefit of nitrogen (N) fixation. In this thesis, intercropping winter oilseed rape (WOR) with legumes (faba bean, blue lupin, common vetch, and clover mixture) will be tested to explore the possibilities of improved N concentration, biomass and weed suppression in organic oilseed rape at the end of autumn growth. Biomass of WOR, legumes, and weeds as well as N concentration were assessed from two field trial studies from two consecutive years, in Örebro county and in Västra Götaland county in November each year. The N concentrations were higher in WOR intercropped with legumes than in the sole crop in 2021, but not in 2022. The biomass of WOR was higher in 2022 than 2021. The biomass of WOR was significantly higher for the sole crop compared to when intercropped with legumes in 2021, but there was no significant difference between the treatments (CCs and row spacing) in 2022. For the sole crop and WOR intercropped with a clover mixture, there was a significant positive relationship between the wet weight of WOR and weeds in 2021, but not for any other treatment during any of the years. There was no significant difference between the CCs regarding any of the three most common weeds. Intercropping with legumes seems to increase the N concentration in WOR, but not the biomass. In 2021, the biomass, competitiveness,and yield of WOR were probably affected by a late sowing date. More biomass of WOR does not seem to lead to less weeds, instead the opposite, probably due to interspecific competition. Changes in soil type within the field of 2022 is likely to have affected the weed prevalence and wet weight of weeds. More data for several years and field trials would be preferable to evaluate the influence of site-specific factors. Yet, there is indications that intercropping could be a promising agricultural practice to increase the N concentration in WOR and may have the possibility to decrease the need for N fertilizers in organic cultivation.